This paper presents a review on residence time distribution (RTD) studies in continuous thermal processing of liquid foods. The theoretical basis of the Danckwerts analysis is summarized, as well as the most important flow models, with special emphasis on tubular systems. Methods for experimental determination, modelling and estimation of RTD are critically described. While main design objectives in continuous thermal processes may be guaranteed by a proper minimum residence or holding time, process optimization requires the knowledge of the residence time distribution. Both concepts are reviewed and discussed. A significant scatter was noticed among published results and the need ,for a systematic work is clear It was concluded that future research should focus on studies at pasteurizationlsterilization temperatures, as well as on studies conducted with real food products or model food systems with FIO~I-Newtoniun flow bchaviour: Furthermore. information relating RTD to processing conditions would be a useful tool for process optimization.

Sucrose acid hydrolysis was studied as a potential chemical time‐temperature integrator to use under pasteurization conditions. A nonisothermal method was used to determine the kinetic parameters of this reaction at different pH values in the range of 0.8 to 2.5 and covering the range of temperatures from 50 to 90G. the nonisothermal method was first validated with the classical two‐step isothermal method at pH 2.5. Kinetic parameters showed to be highly collinear (correlation of 0.99), but it was concluded that the activation energy can be assumed independent of pH and equal to 99 kJ/mole with the preexponential factor being proportional to the H+ concentration. Results are favorable for the future application of this system in the evaluation of pasteurization processes. Since the activation energy was found to be independent of the pH, this system is useful as a TTI for validation of mathematical models, but not so much for monitoring quality factors, except those with an equal activation energy.

ARSTRA CT A systcmutic experimental study on residence time distribution (RTD) in tubular ,jlo~' W(IS pcformcd, covering N wide range of processing conditions. The ,flow oj' wuter in I:arious sections of a tubular continuous thermal processing system was analysed using the classicul Danckwerts upproach. Methylene blue was used as trrrcer and dij$erent constant temperatures (2WIO"C) and jt'ow rates (80-380 li h) were tested. These conditions yielded meun residence times up to 6 min and Reynolds numbers between 1350 cmd 9700. Various models were jitted to the experimental data, and the dispersion model showed to yield the best jit. Peak nnulysis led to both accurate and precise as well as consewative parameters, when compared to other methods of parumeter estimution. Results revealed thnt ,jh{id dispersion in tubular flow (Peclet number) can be related to processing conditions (Reynolds number) by u power law model. Results were compared to published correlations.

The hydrolysis of sucrose in an acid medium was used as a temperature indicator to measure holding temperatures in a continuous thermal-processing unit. From a theoretical analysis, target conversion (v) and errors in both acid concentration and conversion measurements were found to aect signi®cantly the accuracy of predicted temperatures, whereas errors in the determination of the¯uid mean residence time did not show a signi®cant eect. For dierent pasteurisation temperatures (70°C < T < 86°C) and¯ow rates (4800< Re< 11,300), the acid concentration in the medium was adjusted so that dierent extents of reaction could be tested, as the reaction rate was found to increase exponentially with [H ]. Nitric acid solutions were circulated through the unit, a sucrose solution was continuously fed to the entrance of the holding tube and sucrose concentration was analysed at the half way part and at the exit of the holding tube. The temperature of the holding section was then estimated from the measured conversion. These results compared well with thermocouple measurements, with deviations of less than 4°C for conversions between 0.4 and 0.7, whereas greater errors were obtained for both low and high conversions.

The concept of efficiency of a holding tube is essential to guarantee the safety of continuously processed fluid foods. A good prediction of efficiency allows a better control of the processing conditions, guaranteeing product safety while decreasing product quality losses due to overprocessing. Different published methods were compared to data obtained in this work and to reported data, to assess their ability to predict tube efficiency in a range of Reynolds number (Re) covering laminar; transient and turbulent flow. Furthermore, a model assuming dispersed plug flow and a power-law relation between Peclet number (PC) and Re was developed and evaluated. Published models were shown to be, in general, conservative for both laminar and turbulent flow, but often overpredicted the experimental efficiency in the zone of 2100 < Re <4000. Deviations between predicted and experimental values were very considerable (errors from -25% to 90%). The model proposed has proved to be conservative over the whole range, but more accurate (errors ~tp to 15Vrj). 0 1998 Elsevier Science Limited. All rights reservedNOMENCLATURE d Diameter (cm) D Axial dispersion coefficient (m2/s) Length (m) Exponent for the velocity distribution Flow behaviour index, dimensionless Peclet number (Pe = vL/D) Flow rate (l/h) Reynolds number (Re = pvdlp) Residence time distribution Time (min) Minimum residence or holding time (min) Temperature ("C) Velocity (m/s) Volume (I) Efficiency (t: = tmin/t) Experimental efficiency Predicted efficiency Viscosity (poise) Time, dimensionless (0 = t/r) Density (kg/m') Mean holding time (T = V/Q) (min)